Solute-drag forces from short-time equilibrium fluctuations of crystalline interfaces

The design of polycrystalline alloys hinges on a predictive understanding of the interaction between the diffusing solutes and the motion of the constituent crystalline interfaces. Existing frameworks ignore the dynamic multiplicity of and transitions between interfacial structures and phases. Here, we develop a computational framework based on short-time equilibrium fluctuations within molecular dynamics simulations to extract the average stiffness associated with the drag force exerted by the segregating solutes. We show that the random walk of a solute-loaded interface is necessarily non-classical at short timescales due to the confining solute cloud. The behavior due to the much slower stochastic evolution of the cloud can be modeled as an exponentially sub-diffusive Brownian motion in an external trapping potential. The average drag stiffness of the potential yields the net drag force. At longer timescales, the interfacial and bulk forces lead to a gradual recovery of the classical random walk of the interface with a diffusivity set by extrinsic mobility. The short timescales inherent in the response offers a viable high-throughput strategy for extracting drag stiffness and forces, enabling the rational design of alloys to control the microstructural evolution in polycrystals, and in particular, for designing stable nanocrystalline alloys.